1. Field of the Invention
The present invention relates to receiving modules and receivers. More particularly, the invention relates to receiving modules and receivers using Gilbert-cell type mixers.
2. Description of the Related Art
Recently, the Time Division Multiple Access (TDMA) wireless system has been adopted in the Japanese Personal Digital Cellular (PDC) system and in the European Global System for Mobile Communication (GSM). The Code Division Multiple Access (CDMA) wireless system has been adopted in the North American Interim Standard (IS)-95. The TDMA system and the CDMA system are rapidly spreading as wireless communication apparatuses and communication costs are becoming cheaper owing to the efforts of apparatus vendors. A receiver that can be incorporated in both the TDMA wireless system and the CDMA wireless system will be described below.
FIG. 6 shows a block diagram of a conventional receiver. A signal RF received by an antenna 1 is amplified by a low-noise amplifier 2. After this, the signal passes through a high frequency filter 3 and is mixed with a local oscillation signal LO by a mixer 4 to be converted into an intermediate frequency signal IF. The intermediate frequency signal IF passes through an IF band filter 5. Then, a limiter-amplifier 6 amplifies the intermediate frequency signal IF and detects the level of the received signal and provides an output to a demodulator DEM.
FIG. 7 is a circuit diagram of a single-end type mixer that may be used as the mixer 4 in the conventional receiver. A single-end type mixer 50 is constituted of a transistor Q50, capacitors C50 and C51, and a resistor R50. A received signal terminal PRF and a local oscillation signal terminal PLO are connected to the base of the transistor Q50, and an intermediate frequency signal terminal PIF is connected to the collector thereof via the capacitor C50. A control terminal V50 is connected to the collector of the transistor Q50, and the emitter of the transistor Q50 is connected to a ground via the resistor R50 and the capacitor C51. The received signal RF and the local oscillation signal LO are input from the received signal terminal PRF and the local oscillation signal terminal PLO, respectively, and the intermediate frequency signal IF is obtained from the intermediate frequency signal terminal PIF.
When the single-end type mixer is used, isolation between the local oscillation signal terminal and the received signal terminal is not sufficient. In addition, an even-order harmonic occurs due to the non-linear characteristics of the mixer. Thus, it is necessary to simultaneously attenuate a local oscillation signal and a half-image signal (a signal having a frequency that is the sum of the local oscillator frequency plus half the intermediate frequency) causing the even-order harmonic. However, in the PDC system, a received signal is divided into two band signals to be used in a single system. In this case, when the intermediate frequency signal is low, the half-image signal on the high frequency side superimposes itself on the received signal on the low frequency side. As a result, it is impossible to simultaneously attenuate both the local oscillation signal and the half-image signal with a single high frequency filter having a wide pass band. Thus, as shown in FIG. 8, it is necessary to use a pair of surface acoustic wave filters 3a and 3b having steep characteristics adapted to the low and high frequency bands as the high frequency filter between the low-noise amplifier 2 and the mixer 4, and the filters are switched by a selecting switch 7.
For example, in the PDC system a received signal is divided into two band signals so as to use the low frequency band ranging from 810 to 843 MHZ and the high frequency band ranging from 870 to 885 MHZ. When a signal RF of the high-frequency band 885 MHZ is received, the intermediate frequency signal IF is 130 MHZ. In this case, when a local oscillation signal LO of 755 MHZ is used, the half-image signal of 820 MHZ is a disturbance (interference) signal, and superimposes itself on the low-frequency band ranging from 810 to 843 MHZ. As a result, it is impossible to simultaneously attenuate the local oscillation signal of 755 MHZ and the half-image signal of 820 MHZ with the single high frequency filter having the broad pass band ranging from 810 to 885 MHZ. Accordingly, it is necessary to provide two surface acoustic wave filters having the pass bands ranging from 810 to 843 MHZ and 870 to 885 MHZ with the steep characteristics. A select switch is used such that the surface acoustic wave filter having the pass band ranging from 810 to 843 MHZ is selected when a low-frequency side signal is received, and the surface acoustic wave filter having the pass band ranging from 870 to 885 MHZ is selected when a high-frequency side signal is received.
In the conventional receiver, however, since the single-end mixer is used, with the dividing of received signal frequency bands and the lowering of the frequencies of intermediate frequency signals, it is necessary to use a high frequency filter having steeper characteristics. However, it is impossible to have such steeper characteristics even by using surface acoustic wave filters and dielectric filters having good characteristics.
In addition, even if a high frequency filter having such steeper characteristics can be obtained, a select switch is necessary, with the result that the size of the receiver increases.
In order to solve the above problems, the present invention provides a receiving module and a receiver that can be adapted to dividing the bands of received signals and lowering of the frequencies of intermediate frequency signals, in which the sizes and costs of the apparatuses can be reduced.
According to a first aspect of the present invention, there is provided a receiving module including a low-noise amplifier amplifying a signal received by an antenna and a mixer mixing the amplified signal with a local oscillation signal to convert the received signal into an intermediate frequency signal. In this receiving module, the mixer is a Gilbert-cell type mixer.
In addition, the receiving module may further include a multi-layer substrate formed by laminating a plurality of ceramic sheet layers to integrate the low-noise amplifier and the mixer.
According to a second aspect of the present invention, there is provided a receiver including an antenna, a demodulator, a local oscillator, and the above receiving module.
In the receiving module of the present invention, since the Gilbert-cell type mixer is used, sufficient isolation between the terminals can be obtained, and even-order harmonics can be suppressed so as to avoid generating half-image signals.
In the receiver according to the present invention, since the compact and low-cost receiving module is incorporated, the size and production cost of the receiver can be reduced.
Other features and advantages of the invention will be understood from the following detailed description of embodiments thereof.